1. Field of the Invention
The present invention relates broadly to the conveying of sheets or a ribbon of flat glass at elevated temperatures, and more particularly to an improved roll of a fibrous heat resistant material for use in the conveying of such glass, and a method of preparing such a roll. The roll is treated with a solution of potassium sulfate which greatly enhances the lubricity of its surface and binds the fibers together so as to extend its service life and permit its use under temperature conditions not heretofore possible.
2. Description of the Prior Art
In the manufacture of flat glass, a continuous ribbon or sheet of glass is conventionally received from any of the well known plate, sheet or float glass forming devices upon a series of aligned rolls for conveying into and through an adjoining annealing lehr. As received from the forming area, the ribbon or sheet is in a highly heated condition so that its ultimate optical quality can be adversely affected by contact with the conveyor rolls. This is particularly true of glass made by the float process, with which the present invention has particular utility. As shown by U.S. Pat. No. 3,083,551 to Pilkington, glass made by that process leaves the bath chamber in its final useable form with highly polished surfaces. The ribbon is at a relatively high temperature, on the order of 900.degree. to 1400.degree. F. (480.degree. to 760.degree. C.), and thus in a softened condition as it is removed from the bath chamber, and is gradually cooled according to a predetermined pattern as it travels through the annealing lehr. In order to withstand the temperatures to which they are subjected while maintaining a suitable supporting surface, the rolls are generally constructed of an appropriate heat resistant material such as stainless steel or asbestos.
Because of its elevated temperature and the nature of the process, the ribbon is highly susceptible to factors detracting from its ultimate optical quality at this time, particularly during the early stages of the annealing process. Thus, any irregularity or imperfection in the surface of the roll will imprint a corresponding defect in the lower surface of the ribbon while it is in a softened condition. Such irregularities may be caused by scratching of the roll surface, or by the accumulation of deposits thereon, particularly in the initial stages of the annealing process. Thus, in producing float glass tin compounds, generally tin oxides, may be carried from the molten metal bath by the glass ribbon and deposited upon the supporting rolls. These deposits accumulate and eventually cause roll imprinting upon the bottom surface of the ribbon. In an effort to minimize the effects of such deposits and extend the service life of the rolls as well as for other reasons, it has been proposed in U.S. Pat. No. 3,199,966 to Snow et al to treat the glass ribbon with SO.sub.2 gas for providing a lubricating or parting effect between the roll surface and ribbon. However, this treatment can itself result in the formation of sodium sulfate deposits on the rolls which will also eventually mark the bottom surface of the glass. Due to the severe temperature conditions to which they are subjected, the rolls may also tend to develop slight eccentricities, or become out of round, resulting in formation of abrasion or scuff marks on the ribbon of glass as they rotate. Continuous exposure to these operating conditions causes the rolls, particularly asbestos rolls at the hot or furnace end of the annealing lehr, to deteriorate rather rapidly so that they may have a serviceable life of only a few days in some locations, while their use may be entirely ruled out in areas experiencing the most severe operating conditions.
It has been suggested in U.S. Pat. No. 3,853,525 to Gorman that the material-supporting surface of rolls adapted for supporting and conveying glass at elevated temperatures be provided with a coating of boron nitride whose self-lubricating properties will minimize friction between the roll and the glass. U.S. Pat. No. 4,057,410 to Daman et al discloses use of a heat shield of mill board or hot board treated with potassium sulphate for insulating purposes in a float glass producing facility. The thus treated material is said to not mark the glass ribbon should it come in contact therewith.